1. Field of the Invention
The present invention relates to an image forming apparatus operable in a plurality of operation modes including a monochrome printing mode and a color printing mode.
2. Description of Related Art
In an electrophotographic color image forming apparatus, a plurality of units, each of which includes a photoconductor drum, a development roller, a transfer roller and other components, are provided for a plurality of colors, and supplied with toner of corresponding colors to form a toner image of each color one after another on an intermediate transfer belt or a recording sheet being transported. In the following description, the intermediate transfer belt and the recording sheet are collectively referred to as “transfer medium” or “medium” where appropriate.
To do monochrome printing in the color image forming apparatus, no toner other than the toner in monochrome (typically in black) is required, and thus development devices provided for the other colors may be put out of operation, for example, by separating the development rollers from the corresponding photoconductor drums so that degradation of the toner of these other colors are reduced.
On the other hand, generally speaking, not all the toner on the surfaces of the photoconductor drums will be transferred from the photoconductor drums to the transfer medium, and some toner may remain on the surfaces of the photoconductor drums. Thus, it would be desirable to remove the remaining toner. For that purpose, a cleaning unit having a blade and/or a roller may be provided, as is often the case. Alternatively, a method of collecting the remaining toner on the photoconductor drums by the development rollers and remixing the collected toner back into the toner in the development devices may be adopted. This type of cleaning method is sometimes called “cleanerless” method.
However, if the cleanerless method could be adopted in the image forming apparatus configured to separate the development rollers from the corresponding photoconductor drums except the photoconductor drum used in monochrome printing during the monochrome printing, the toner supplied for the monochrome printing, transferred to the transfer medium during the monochrome printing and adhered (“reversely transferred”) to the photoconductor drums located downstream of the photoconductor drum for the monochrome printing would not be collected by the development rollers separated from the corresponding photoconductor drums. This would disadvantageously produce a ghost image derived from the retransfer to the transfer medium, of the reversely transferred toner image on the photoconductor drum.
Thus, there is a need to reduce the possibility of formation of a ghost image in an image forming apparatus adopting a cleanerless method and operable in a monochrome printing mode in which the development rollers are separated from the corresponding photoconductor drums other than that to be used for the monochrome printing.
The present invention has been made in an attempt to address the aforementioned problem in prior art.